There is a known thermomagnetic recording system which uses a layer of soft magnetic material having an easy axis of magnetization normal to the layer surface on which the thermomagnetic recording is performed by irradiating a laser light thereon. The system is disclosed in the publicated document of Japanese patent application unexamined No. 158005/1982 (Japanese patent application, No. 45232/1981) filed by the present applicant. The layer of soft magnetic material having an easy axis of magnetization normal to the layer surface used therein is a layer made of soft magnetic material, for example, YSmCaFeGe-series garnet such as (YSmCa).sub.3 (FeGe).sub.5 O.sub.12 or the like, having uniaxial magnetic anisotropy strong in the direction normal to the layer surface and an easy axis of magnetization normal to the layer surface. The layer of soft magnetic material having an easy axis of magnetization normal to the layer surface is required to have such soft magnetic property that when this layer is used as a magnetic recording medium, the diameter of information bits to be written therein is practically determined only by a bias magnetic field, and the coercive force thereof is desired to be less than about 3 Oe, preferably about 1 Oe or below. This layer of soft magnetic material having an easy axis of magnetization normal to the layer surface is formed by growing on a crystalline substrate of rare-earth gallium garnet such as non-magnetic gadolinium gallium garnet (GGG) by the liquid phase epitaxial (LPE) growth of YSmCaFeGe-series garnet crystal or the like. The writing an information on that layer is carried out such that a bias magnetic field of a predetermined strength is first applied to the layer of the soft magnetic material so as to make the layer magnetized to have a single magnetic domain over the whole visual field thereof and magnetization directed to the direction perpendicular to the layer surface. Then, under this state, when an optical pulse is incident on the layer surface being focussed thereon, the writing of an information on the layer can be performed. The bits thus written are cylindrical magnetic domains each having a predetermined diameter and the magnetization directed to the direction opposite to the applied bias magnetic field.
The strength of the bias magnetic field applied to the layer of soft magnetic material so as to produce a single magnetic domain therein over the whole visual field thereof is selected in a range from the run-out magnetic field to the collapse magnetic field of the material, for example, between 57 Oe and 73 Oe for the above-mentioned LPE layer of (YSmCa).sub.3 (FeGe).sub.5 O.sub.12. Because the coercive force, particularly magnetic wall coercive force of the layer of the soft magnetic material is extremely small and hence the bias magnetic field applied thereto can be small, the bias magnetic field applying means may be a small solenoid coil, rubber magnet or the like.
The reading-out of the information from the magnetized layer on which the recording was made is carried out such that a light, for example, a laser light is linearly polarized by a polarizer and then irradiated on the record medium. If so, when the linearly polarized light passes through the magnetized layer, it is subjected to the rotation of plane of polarization by the Faraday-effect. Thus, if this light caused in rotation is applied through an analyzer into a photo-detecting means, the output corresponding to the information bits is produced therefrom and hence the reading-out of the information is performed.
In such thermomagnetic recording system, however, the garnet film used as a recording layer presents the transmissivity for a light of wavelength longer than about 530 nm and hence the light of such long-wavelength can almost not be absorbed by the film having a thickness of 10-.mu.m or below. Therefore, in this thermomagnetic recording system, an Argon laser having a wavelength of, for example, 488 nm is used as the light source thereof.
The present invention is to obviate the defects in the thermomagnetic recording and reproducing system mentioned above and to enable that even a semiconductor laser having a long wavelength which could not be absorbed by the magnetized film, can be used to record informations on the layer. As a result, the thermomagnetic recording and reproducing system of the invention is capable of increasing a recording density and of logical calculation.